Fermented foods for the microbiome: what's actually proven

Fermented foods for the microbiome are yogurt, kefir, sauerkraut, kimchi and other foods fermented with live cultures. Research links eating them regularly with a more diverse gut microbiome, so they're worth including in your diet in small but steady amounts.

Interest in the microbiome has moved these foods from "national tradition" into mainstream healthy eating — and not without scientific grounds.

What are fermented foods?

They're foods in which bacteria or yeasts have partly processed the sugars in the raw ingredients. This changes the taste and texture, and some products keep their live cultures.

Fermentation is one of the oldest ways to preserve food. Almost every cuisine in the world has its own examples, from kefir and ayran to miso and kimchi.

During fermentation, a food can become easier to digest, gain a tangy taste and keep longer. That's why fermented dishes have been part of the winter diet in many countries for centuries.

Best fermented foods for gut microbiome: a list for everyday eating

The most accessible options can be found at a regular grocery store or made at home.

• Yogurt and kefir with live cultures.
• Sauerkraut and kimchi — unpasteurized, since pasteurization kills the beneficial bacteria.
• Miso and tempeh — soy foods from Asian cuisine.
• Kombucha — fermented tea, in moderate amounts.
• Some aged cheeses.

When choosing yogurt or kefir, look for "live and active cultures" on the package and check the sugar: a plain version without additives can be sweetened with berries yourself.

Not everything that looks pickled contains live cultures: vinegar pickles and pasteurized canned goods are something else. To tell them apart, use the skill from "How to read food labels".

How do fermented foods help the gut, according to research?

Studies cited by Harvard Health and the Harvard School of Public Health link regularly eating fermented foods with positive changes in microbiome diversity and some inflammation markers.

It's important to understand the limits of this data: we're talking about associations and changes in markers, not a guaranteed effect for everyone. The microbiome is individual, and different people can react differently to the same food.

For more on how gut health is linked to energy and mood, see "Gut health and wellbeing", and for the difference between live cultures in food and supplements, "Probiotics and the gut microbiome".

How fermented foods improve your health: benefits and downsides

For most people the benefits outweigh the downsides, but these foods have some quirks worth knowing about.

• Sauerkraut, kimchi and miso can be quite salty — important if you're watching your salt.
• Sweetened yogurts and some kinds of kombucha can contain a lot of sugar.
• In some people, a sudden jump in portions causes temporary bloating.

So the best option is plain products without extra sugar, in moderate portions and as part of a varied diet, not as a "miracle food."

How to add fermented foods without discomfort?

Start with small portions and increase gradually. Adding these foods to your diet suddenly sometimes causes temporary bloating.

• Choose one food and eat a little of it for several days in a row.
• Add the next one once the first has become familiar.
• Pair them with fiber: vegetables, beans and whole grains serve as "food" for beneficial bacteria.

They're easy to work into snacks — for example, kefir or plain yogurt with berries. For ideas, see "Snacks for energy".

How to make fermented foods at home?

The easiest start is sauerkraut: shredded cabbage is rubbed with salt, packed tightly into a clean jar and left to ferment at room temperature for a few days.

• Use clean equipment and make sure the vegetables stay covered with their juice.
• Taste it every day to catch the tanginess you like, then move the jar to the fridge.
• Anything that grows mold or smells unusual is best thrown away.

Fermented foods aren't a magic fix but a pleasant, accessible addition to a varied diet rich in plant foods.